Galaxies are not solitary objects - usually, they cluster together. Our
own galaxy for instance, the Milky Way, is part of a small cluster
called the local group of galaxies. The next-closest large galaxy cluster is the Virgo cluster.

gamma rays

The most highly energetic variety of electromagnetic radiation , mit above a quintillion oscillations per second, corresponding to wave-lengths of less than a hundredth billionth of a meter.

In a broader sense, gas is also used to denote other mixtures of freely careening particles, for instance in the case of the electron gas whose pressure stabilizes a
white dwarf against further collapse.

For information about the concepts and applications of this theory, we recommend the chapter general relativity of our introductory section Elementary Einstein. Further information about many different aspects of general relativity and its applications can be found in our section Spotlights on relativity.

In classical mechanics,
the rotation axis of a gyroscope on which no external forces are acting
will remain constant - a useful property that has found applications in
navigation. However, in the presence of spacetimecurvature,
this is no longer true - the axis direction of a gyroscope in free fall
will change over time; an effect predicted by Einstein's general theory of relativity.

geometry

That part of mathematics concerning itself with surfaces or more general spaces as well as objects defined on such spaces, such as points or lines as well as the objects constructable from points and lines, such as triangles.

A system of satellites and mobile receivers that makes it possible to determine each receiver's position with high accuracy. Used by pilots, truckers, car drivers and hikers world-wide, it is an industrial application of Einstein's theories of special and general relativity: Without taking into account the effects predicted by these theories for moving clocks in a gravitational field, there would be errors of roughly 10 kilometres per day of operation in the determination of positions on earth.

Gowdy spacetimes (or universes) are simple expanding model universes. In contrast with the better known Friedmann-Lemaître-Robertson-Walker universes (the basis for the big bang models), Gowdy universes are not homogeneous. Instead, they are filled with a regular pattern of gravitational waves. A Gowdy T3 universe is the simplest kind of Gowdy universe, in which space has the shape of a three-dimensional torus.

Compared with other fundamental constants, G is known only to a comparatively low accuracy.

gravitational field

The totality of all gravitational influences that one or more massive objects can exert on bodies in their vicinity.

More precisely: At every location in space, the gravitational field is defined as the acceleration that a small test particle present at that location would feel due to the gravitational forces
of the masses around it.

gravitational lens

In Einstein's general relativity, gravity necessarily acts not only on material bodies, but also on light - light passing a massive body is deflected. This deflection can be so strong that light of one and the same cosmic object reaches an observer along multiple paths - corresponding to the observer seeing multiple images of that object in the sky. Masses that, in this sense, act like very special optical lenses are called gravitational lenses.

According to general relativity, light flying away from a massive body (or other source of gravity) experience a redshift - its frequency
decreases and the light becomes less energetic. On the other hand,
light flying towards a massive body gets blueshifted - its frequency
and energy increase.

gravitational wave astronomy

Nascent discipline of astronomy which aims at using
gravitational waves
to gain information about cosmic objects or the cosmos as a whole - for
instance about what's happening in the core region of a
supernova, about
neutron star or about the heated past of our universe.

So far, though, scientists are still working on the first direct detection of gravitational waves using highly sophisticated gravitational wave detectors, after which gravitational wave astronomy is hoped to begin in earnest.

Circle on the surface of a sphere whose center coincides with that
of the sphere itself. On the globe, the equator is a great circle,
while every meridian corresponds to half of a great circle.

If you want to move on a spherical surface in the straightest
possible way, choose a path along a great circle - in the language of
mathematics this is equivalent to saying: great circles are
geodesics of a spherical surface.